The Science of Sleep: What It Is, How It Works, and Why It Matters

By Wallace B. Mendelson

“Attractive, artistic, informative, engaging, and lucidly written . . . Mendelson provides an excellent introduction to sleep science and sleep medicine.” —Sleep and Vigilance

We often hear that humans spend one third of their lives sleeping—and most of us would up that fraction if we could. Whether we’re curling up for a brief lunchtime catnap, catching a doze on a sunny afternoon, or clocking our solid eight hours at night, sleeping is normally a reliable way to rest our heads and recharge our minds. And our bodies demand it: without sufficient sleep, we experience changes in mood, memory loss, and difficulty concentrating. Symptoms of sleep deprivation can be severe, and we know that sleep is essential for restoring and rejuvenating muscles, tissue, and energy. And yet, although science is making remarkable inroads into the workings and functions of sleep, many aspects still remain a mystery.

In The Science of Sleep, sleep expert Wallace B. Mendelson explains the elements of human sleep states and explores the variety of sleep disorders afflicting thousands of people worldwide. Mendelson lays out the various treatments that are available today and provides a helpful guide for one of life’s most important activities. By offering the first scientific yet accessible account of sleep science, Mendelson allows readers to assess their personal relationships with sleep and craft their own individual approaches to a comfortable and effective night’s rest.

Addressing one of the major public health issues of the day with cutting-edge research and empathetic understanding, The Science of Sleep is the definitive illustrated reference guide to sleep science.

• Language: English
• Publisher: Open Road Integrated Media
• Release date: Feb 14, 2018
• ISBN: 9780226387338

Book preview

INTRODUCTION

Sleep means different things to different people, and indeed its meaning differs in the same person at various times. I remember as a boy going to bed on Christmas Eve, excitedly anticipating the presents awaiting me in the morning, and thinking that since I would soon be asleep, the time would seem to pass in an instant. Then the presents would be mine. Not surprisingly, that sort of thinking led instead to a long period of unhappy wakefulness. The opposite can occur as well, as in the song The Green Green Grass of Home, in which sleep is a time of escape into happy memories, in contrast to the very unfortunate events awaiting the sleeping prisoner in the morning. For others, sleep can become a kind of testing ground: a person who prides herself on always being the best at whatever she does can view good sleep as a challenge, something she has to work at—the result, paradoxically, being poor sleep. It can also be a time of anxiety. People for whom it is important to feel in control of things can find it worrisome to have a period each night in which they seem more vulnerable and not in charge. For others, sleep can be a time of getting a glimpse of the real world; I have had patients who say that their dream experiences during sleep seem so much more real and meaningful than what they awaken to in the morning.

Sleep is also tied to the notion of restoration. After a good night’s sleep a healthy person awakens with a sense of vitality, of readiness to face the new day. As we will discuss later, no one is certain what this entails physiologically—it is not just a simple matter of increasing metabolic energy stores—but its presence (or absence) plays a role in what we think about sleep. Related to this is the notion of sleep as a pleasurable experience, something to look forward to. Sadly, for many people the opposite is true. The genesis of this is not always clear. Some think that a lifelong feeling that sleep is an unhappy time is a derivative of childhood experiences, in which the more typical learning association of sleeping with pleasure did not take place. Others view this as a disorder of the amount of brain chemicals that normally bring on sleep. Another view is that it may result from habits in which bedtime is used for behaviors incompatible with sleep, such as worrying and planning tomorrow’s battles.

Sleep is inextricably tied to a world of alternating day and night, and our bodies have developed mechanisms to time our waking and sleep accordingly. Sometimes this timing can go astray, either due to behaviors such as engaging in shift work or flying long distances, or due to inherent problems of the body clock. These in turn can lead to difficulties with sleeping, or at least with sleeping during the traditional hours allocated for it.

Sleep can also be a kind of social behavior, inside the species, or across species (for instance when sleeping with a pet). We often use the euphemism of sleeping together to refer to another kind of activity that can take place in bed, but this kind of delicate phraseology can obscure another aspect, which is that repetitive sharing of the sleep experience may play a role in a couple bonding together.

There is also a sense that sleep is important to health, both physical and mental. Sleep which is curtailed or disrupted can lead, for instance, to a predilection to diabetes and related disorders. It seems to be important for the formation of long-term memories. This suggests, for instance, the futility of students doing all nighters of studying. It turns out that getting a good night’s sleep may be the most helpful thing in preparing for an exam in the morning. One of the great believers in a good night’s sleep, incidentally, was Alexander the Great. In 331 BCE, before the critical battle of Gaugamela in which he overwhelmed a vastly larger Persian army on their own territory, he slept so deeply that his officers became worried and had to awaken him. He got up, put on his armor, and went on to an outstanding victory which set the stage for conquering an empire.

In this book we will present the scientific understanding of sleep, beginning by describing its basic processes and how to measure them. It will be seen that sleep results from the careful orchestration of a variety of physiologic processes. As in any complex mechanism, sometimes things go awry, in this case resulting in clinical sleep disorders which are experienced as insomnia, excessive sleepiness or undesirable behaviors during sleep. We will describe some of these disorders, and some of the treatments that are available. This information is not a substitute for medical evaluation. If you think you may have a sleep disorder, you should consult your doctor for evaluation and possible referral to a sleep disorders center. It is hoped that, armed with the information provided here, you will be better able to understand and discuss what is happening, and to make more informed choices in conjunction with your doctor.

Just as sleep is a universal human behavior, so is human curiosity and the desire to know more about ourselves. A number of men and women devoted themselves to learning more about sleep, long before sleep studies became an established scientific discipline. They came from a variety of unlikely backgrounds—a World War I cavalryman and a fighter pilot, for example. One was looking for something entirely else, the basis of a supposed psychic energy which might let people communicate across long distances, and ended instead with the groundbreaking discovery of the human electroencephalogram. Another was a doctor faced with treating patients in a worldwide epidemic of encephalitis, who recognized a pattern to the parts of the affected brains—and learned from it the basic structures making it possible for us to be awake or asleep. Another had made his fame developing a method to measure the speed of projectiles for the Army, but curiosity led him to measurements of many other kinds of things, including electrical waveforms during the human sleep stages. If you, the reader, have picked up this book, it sounds like you, too, have curiosity about how things work, and it is my hope that here you will learn more about how we sleep and wake.

Chapter One

HUMAN SLEEP

We can measure our lives on many different levels. A life, for instance, could be considered the period in which a person’s heart beats a certain number of times (perhaps 3 billion), or enjoys Saturday nights (about 3,600), or experiences periods of peacefulness (all too few), or feels the joy of love (hopefully at least once). In the same way, sleep, which occupies perhaps one-third of our existence, can be viewed on a number of different levels. It is a process that can be measured physiologically while at the same time understood as a psychological experience, and even a social behavior. In this book we will explore some of these aspects of sleep. Human sleep is a reversible period of decreased consciousness and responsiveness, comprised of two distinct states known as rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Although the modern era of sleep research began in the 1950s with the description of REM sleep, its roots go back to the 1920s with the discovery of the human electroencephalogram, or EEG. Our growing understanding of sleep has been influenced by some remarkable individuals who (sometimes while looking for something else) made crucial observations about sleep, by developments in psychology and technology, and even by world events.

WHAT IS SLEEP?

The many qualities of sleep

Sleep is a period of recurring behavioral quiescence, which has several qualities, including decreased awareness of and responsiveness to the environment, diminished consciousness, and the rhythmic appearance of certain physiologic patterns (stages). It tends to occur at particular parts of the 24-hour day–night cycle, and at customary locations, both depending on the particular species and environment. It is reversible, distinguishing it from coma or ongoing anesthesia. It is also self-regulating: if one is deprived of sleep, there will be a drive to have increased recovery sleep to make up for the loss. It is necessary for life, and is present in all mammals. These qualities will be discussed later in the chapter.

DIMINISHED CONSCIOUSNESS

The nature of diminished consciousness in sleep is by far the hardest to express, since consciousness itself is so poorly understood. When we speak of consciousness, we refer to our experience of self and the world. Perhaps even more fundamentally, consciousness can be defined as our mode of access. We can additionally speak of acts of consciousness, such as perceiving, willing, imagining, and the contents of these acts, such as perceiving a sunset, willing a meeting, or imagining a good outcome.

Other efforts to characterize consciousness are helpful. For example, consciousness has been described as a paradoxical state in which a person is simultaneously a subject who can experience things and an object perceived by oneself. Generally most of us recognize this when we speak of ourselves in both ways simultaneously, for instance when we say, Sometimes I have to remind myself that... or I owe it to myself to... . In such phrases we recognize that we are both a being having experiences and at the same time we can picture ourselves as objects.

Additional qualities of consciousness include consciousness as subjective and private (not available to another person); unitary (experienced by a single person); and characterized by a subjective feel of each experience, a what it is like aspect. The American philosopher Thomas Nagel (1937– ) illustrated the feel which is essential to consciousness in the following manner: although we may study and understand the neurophysiology of a bat, we can never know how the world is experienced by a bat. Some authors argue that a subjective experience such as consciousness cannot profitably be studied by traditional scientific techniques; others believe that the fact that a phenomenon is experienced subjectively does not mean that it cannot be explored objectively.

New developments in technology, including the use of brain-imaging studies, are beginning to advance our understanding of the physiology behind consciousness, and researchers have developed a number of models of how it may occur. In addition to looking at normal sleep, neuroimaging studies of people who have been given hallucinogens such as psilocybin have contributed new theories of consciousness based on the notion of entropy—the degree of order and disorder in neural connections.

During sleep one becomes less aware of the surroundings; this is one of the qualities that distinguishes sleep from quiet wakefulness. An extreme case would be someone who falls asleep at the wheel, consequently running a red light. On the other hand, this process is not absolute; it is clear that we can process, and act on, sensory information during sleep. A new parent, for instance, can sleep through the noise of a truck driving by the house, but awaken quickly at the sound of the newborn baby crying. Similarly, in a laboratory situation, the volume of a sound required to wake a person up (the auditory arousal threshold) is much higher for a meaningless sound such as an electronic tone compared to a meaningful stimulus such as a phone ringing or hearing one’s name called. A sleeper, then, is able to process information about an incoming sound, and determine whether it is important or not. Again, the arousal response is dependent on a variety of factors including the sleep stage, the duration of wakefulness before sleep, and how far into sleep one is when the sound occurs. Some individuals are more likely to be awakened by low volume noises, and hence are considered to be light sleepers. Interestingly, people with insomnia often have a normal auditory arousal threshold, suggesting that insomnia is different from just light sleep. Some sleeping pills increase the auditory arousal threshold, raising the possibility that the medicated sleeper will not be aroused by, for instance, a smoke alarm.

Time-lapse videos show that two sleepers adjust their positions in response to the other’s movements (whether the other sleeper is a pet or another human). In one film, for instance, a man sleeping with a cat on the bed may turn on his side, with the cat then responding by settling comfortably in the warm nook behind his knees. In co-sleeping humans, an elbow in the ribs may result in an adjustment of the second person’s position. Thus, although sensory input is diminished during sleep, this is not absolute, and indeed during sleep we are able to take in and act on information to some degree.

SLEEP STUDIES

This composite picture shows a subject undergoing a sleep recording across the night. The scientific study of sleep continues to reveal more and more insights into what happens while we are asleep, and how it influences our waking lives.

HOW IS THE ELECTRICAL ACTIVITY OF THE BRAIN MEASURED?

Introduction to the Electroencephalogram (EEG)

Sleep is comprised of rhythmically recurring sleep stages, which are defined by looking at brain waves, or an electroencephalogram (EEG), in conjunction with the electrooculogram (EOG), which records eye movements and the electromyogram (EMG), a measure of muscle activity.

The discovery of brain waves

Before the sleep stages are described, it is useful to look at how the brain waves were discovered, and what they are like. It was discovered in the 1820s that electrical current could cause the needle of a magnetic compass to fluctuate, and that this effect could be multiplied by the use of coils of wire. An instrument based on this observation was known as a galvanometer, named after Luigi Galvani (1737–98), an Italian physician and biologist who in 1791 observed that electrical currents could cause the limbs of a dead frog to twitch. This was one of the first observations that electrical currents might be involved in biological processes. Some years later, Richard Caton (1842–1926), a Scottish physiologist, used a galvanometer to detect electrical current from the brains of dogs and apes. In 1875 he reported that the current differed at various times, increasing in strength during sleep and preceding death, then disappearing upon death.

A half century later, Hans Berger (1873–1941), a German psychiatrist, made the next great advance. Berger had been a mathematics student who dropped out and enlisted in the cavalry. One day, he was thrown from his horse, landing in the path of an oncoming cannon carriage, which barely managed to stop at the last moment. At the same time that Berger had this life-threatening experience, his sister living some distance away is said to have had a sudden sensation that he was in danger, and urged her father to contact him. Berger was so struck by this apparent psychic energy alarming his sister, that he went on to devote his life to exploring the brain and how objective measures of its activity might relate to subjective psychic processes. In 1929, he described recordings of the electrical waves in humans that had been reported by Caton in animals, and developed a way to record them on moving strips of paper. He coined the term electroencephalogram to describe his new discovery. He showed that the waves differed in waking, in sleep, and in anesthesia, and that sharp spiking patterns appeared during epileptic seizures. He described the alpha rhythm in a subject resting with closed eyes, and its disappearance and replacement by faster beta waves when the eyes were opened. These waveforms over the years have been divided into several wavebands according to their frequency (number of waves per second). Other information used in describing them involves their shape, amplitude (a measure of their energy), and location on the head (see also Features of an oscillating system, shown here). Subsequent to Berger’s work focusing on alpha and beta, a series of EEG bands have been further defined and are generally recognized to this day.

MAJOR EEG BANDS

Delta waves (0.5–4 Hz)

These slow waves have a frequency range of 0.5–4 cycles per second (known as cps or Hz). Our interest in them from the point of view of sleep is that they are characteristic of stage N3—also known as slow-wave sleep (SWS), or stages 3 and 4. In clinical EEG work in waking patients, in distinction from sleep studies, they can be associated with the presence of lesions in local areas of the brain, or can appear in a widespread manner in diffuse disorders.

Theta waves (4.5–8 Hz)

These waves appear during lighter sleep. In waking clinical EEG work they are considered a sign of sleepiness, increasing with the duration of wakefulness. They can also appear during hyperventilation. In sleep studies, they are an important rhythm in stage N2 ( stage 2) sleep.

Alpha waves (8.5–12 Hz)

Also known as Berger waves, these are best seen occipitally (in the back of the head), and are manifested in a relaxed but awake person with eyes closed. As mentioned before, they greatly decrease when the eyes are opened. When eyes are closed in an awake person, they decrease as one becomes sleepy.

Beta waves (12.5–30 Hz)

Beta waves, which are more evident anteriorly (toward the front of the head), are often divided into irregular, disorganized, low amplitude and organized waveforms. The former is primarily seen in an awake person who is actively thinking, concentrating, or feeling anxiety. The latter is seen in some illnesses or as a result of some sedatives including barbiturates or Valium-like drugs (benzodiazepines). All EEG bands (including beta) can be viewed visually or measured electronically. The amount of electronically measured beta activity is sometimes considered a measure of arousal of the cerebral cortex.

THE SLEEP STAGES

Initial discoveries

The next step in the evolution of understanding sleep was the recognition that EEG brainwaves, when combined with other kinds of physiological information, could be used to identify rhythmically recurring, discrete sleep stages. The first description of these sleep stages was made by the American scientist Alfred Loomis (1887–1975). As a young man in the army he developed the Aberdeen Chronograph, a system for measuring projectile velocity by firing a bullet through revolving paper-covered aluminum disks. He went on to a successful career in investment banking. Becoming restless once again, and still remembering his success with the chronograph, he turned his attention to developing radar for military purposes and for ground control during the landing approach of aircraft. He was fascinated with the measuring of waveforms and, among the many projects at his laboratory at Tuxedo Park, New York, was the study of sleep. Using a large 8 foot (2.4 m) diameter recording drum, he described in 1937 a series of five discrete recurring sleep stages during the night, which he rather unpoetically designated as stages A–E. In terms of later development, stages A and B correspond roughly to what was later called stage 1, C corresponded to stage 2, and D and E resembled slow-wave sleep. All together they correspond to what we now call non-rapid eye movement (NREM) sleep.

REM SLEEP DURING THE NIGHT

RAPID EYE MOVEMENTS

An example of EOG (electrooculogram) channels in a polysomnogram showing the rapid eye conjugate eye movements that occur during REM sleep. The discovery of REM sleep represented an exciting breakthrough in sleep research.

The discovery of rapid eye movement (REM) sleep

The next big development, which in effect ushered in the modern age of sleep research, occurred in the early 1950s. Nathaniel Kleitman (1895–1999), a physiologist at the University of Chicago, had been interested in eye movements and blinking as a marker of sleep onset and depth of sleep, as well as possible rhythmic behaviors, in infants. He enlisted the aid of a graduate student, Eugene Aserinsky (1921–98). Following observations in infants, they adapted the technique of the electrooculogram (EOG) for continuous use in sleep of children and adults. In the process of doing so, they observed the periodic appearance of vigorous and jerky ocular activity. This new stage, known as rapid eye movement (REM) sleep, was characterized